Various plastic articles, such as plastic closures for containers, have been formed by a compression molding process. Some compression molding machines have a plurality of tools mounted in a circumferential array on a rotatable turret in a plurality of opposed coacting pairs. The tools of each pair carry opposed male and female mold sections that when closed together form a cavity mold for compression molding the desired articles. The turret rotates adjacent a machine frame that carries cams for moving the tools of each pair toward each other during a portion of each revolution of the turret for compression molding articles between the tool pairs, and away from each other during another portion of the each turret revolution for releasing articles molded between the tools and to receive fresh charges of plastic into the mold cavities.
So that the opposed pairs of tooling can move relative to each other and the turret, the tooling is typically mounted on elongate rods by bushings and bearings to permit slidable movement of each half of a tooling pair relative to the other half of the pair. A defined clearance is required between each rod and its associated bushings and bearings to permit relative sliding movement of the mold tooling relative to the rod without undue friction and typically, to receive a lubricant to facilitate that relative movement. The required clearance reduces accuracy in the alignment of the opposed halves of the tooling pair and can increase wear of the bushings bearings, rod and other tooling components, and the need to maintain lubrication increases the maintenance burden for the apparatus.
After the opposed halves of the tooling pair are mated together to form a charge of plastic into its desired shape. It is desirable to lock the mated halves of the tooling pair together so that the desired compression molding force is maintained between the halves of the mated tooling pair without having to continually apply an external force to maintain the tooling halves together. However, the plastic being molded will shrink as it cools and cures, and it is desirable to reduce the size of the mold cavity in proportion to the shrinkage encountered during cooling of the plastic to maintain a desired pressure within the mold cavity and acting on the plastic material throughout the compression molding process. In view of the relatively high pressure and force utilized during the compression molding process, it is also desirable to provide a mechanism or assembly to protect the tooling and compression molding machine in general should a failure occur, such as may happen if a previously formed part is not adequately removed from the mold tooling, or if an overly large plastic charge is delivered into a mold cavity, for example.